This invention relates to low-voltage, high-current inverter transformers, and more particularly relates to an improved winding configuration for such transformers.
High frequency switching circuits have been used to advantage in power conditioning apparatus for many decades. A major advantage comes from the fact that high-frequency transformers are much smaller, lighter, cheaper and more efficient than low-frequency units of the same power rating. In the particular range of applications involving rectified outputs at low voltage and high current, the efficiency of the rectifiers and rectifier circuit are also important considerations. The two commonest rectifier circuits are the full-wave bridge rectifier (FWBR) and the full wave circuit center-tapped rectifier (FWCTR), shown in FIGS. 1a and 1b, respectively.
The function of both circuits shown is to rectify the AC (alternating current) provided from the transformer, by means of the rectifiers or diodes, to DC (direct current) which is then delivered to the load. In the case of the bridge rectifier, the AC current from the transformer passes through two diodes in series in order to flow to the load. In the case of the center-tapped circuit the AC current from the transformer passes through only one diode to flow to the load. In either case, the peak and average current is the same. Given that the forward drop of each diode is the same, the power loss in the diodes as a whole in the bridge circuit is twice that of the center-tapped circuit. Typically, the forward drop of a diode is on the order of 1 to 2 volts. If the output voltage of the rectifier is low, this has a significant impact on the efficiency. The impact is much more for the bridge rectifier than for the center-tapped circuit. For example, if the output voltage is 5 volts and the diode drop is 1 volt, the rectifier efficiency for the bridge is 5/(5+2)=71%; and for the center-tapped circuit is 5/(5+1)=83%.
There are other differences between the two circuits. The peak inverse voltage rating of the diodes in the full wave center-tapped circuit must be about twice that of the diodes in the bridge circuit. However, there are only two diodes in the FWCTR opposed to four in the FWBR. The transformer winding for the FWBR is half the voltage rating of the center-tapped winding and has a better utilization factor. However, considering all factors, the center-tapped circuit is the better choice for low-voltage high-current applications. Another consideration is the fact that rectifiers or diodes for high frequency applications are limited in the available maximum current rating to a few tens of amperes. This requires the use of parallel rectifier circuits in high current applications. The parallel circuits must match closely in electrical characteristics to insure that current will share equally, hence the transformer secondary windings of each parallel circuit must be matched. The best way to accomplish this is to wind identical secondary sections (Sy) side-by-side over the primary (Py) as shown in FIG. 2.
From the coil diagram in FIG. 2 it is obvious that manufacturing the several secondary windings is a complicated procedure in that the start leads must be routed up between each coil section. This requires that each section be individually wound. In addition, the procedure is more complicated if the secondary is center-tapped and each side of the winding from the center-tap is to be identical. The present invention discloses a new winding configuration and winding procedure that provides for the simultaneous winding of identical sections, either center-tapped on non-center-tapped in a single operation.